1. Field of the Invention
The present invention relates to an image heating apparatus and an image forming apparatus configured to heat a recording medium at a nip portion formed between a belt member supported by a supporting structure and a roller member.
2. Description of the Related Art
An image forming apparatus configured to form a toner image on an image carrier, to transfer the toner image directly or through an intermediary of an intermediate transfer body to a recording medium, and to fix the image to the recording medium by heating and pressing the recording medium on which the toner image has been transferred at a nip portion of a fixing apparatus is widely used in general.
Hitherto, a belt heating-type fixing apparatus provided with a nip portion formed by putting a roller member in contact with a belt member supported by a support structure has been put into practical use as disclosed in Japanese Patent Application Laid-open No. 2010-190967 for example. This belt heating-type fixing apparatus is configured such that the nip portion is heated by a heating element assembled in the support structure through the intermediary of the belt member, and such that the temperature of the nip portion is controlled by using a temperature sensor assembled in the support structure.
By the way, there is a case when the pressurizing conditions of a longitudinal center part and of longitudinal end parts of the nip portion change due to time-dependent change and others of a rubber material used for the roller member in the belt heating-type fixing apparatus described above. Then, if a greater amount of pressure is applied eccentrically to the longitudinal end parts of the nip portion due to such changes, the quantity of heat to be applied to an image part that passes through the longitudinal center part of the nip portion may become insufficient, causing uneven fixation. If a greater amount of pressure is applied eccentrically to the longitudinal center part of the nip portion, the conveying speed of the longitudinal center part of the nip portion may increase more than that of the longitudinal end parts, causing wrinkles at a rear end of a recording medium.
In a case of estimating the temperature of longitudinal end part of a nip portion by a temperature sensor disposed at longitudinal end part of a support structure as disclosed in Japanese Patent Application Laid-open No. 2000-250374, a relationship between a detected temperature and an actual temperature of the nip portion varies if pressurizing conditions of the longitudinal center part and of the longitudinal end parts change. As a result, the sensor may overestimate the temperature of the nip portion, decreasing the productivity of the image forming apparatus unnecessarily. Or, the sensor may underestimate the temperature of the nip portion, decreasing the durability of the belt member.
Then, Japanese Patent Application Laid-open No. 2002-174987 has proposed to estimate the pressurizing condition of the entire nip portion by detecting a rotational speed of the roller member forming the nip portion and to change a target temperature of the roller member to be adjusted based on this estimated result.
Japanese Patent Application Laid-open No. 2005-301070 has also proposed to estimate the pressurizing condition of the entire nip portion by measuring an accumulated number of recording media heated at the nip portion or an accumulated used time of the belt member and to change the target temperature of the roller member to be adjusted based on this estimated result.
Unfortunately, the methods disclosed in Japanese Patent Application Laid-open Nos. 2002-174987 and 2005-301070 have had a problem that even though they enable an evaluation of the pressurizing condition of the entire nip portion, they are unable to determine whether the pressure is applied eccentrically on part of the longitudinal center part or the longitudinal end part of the nip portion.